\documentclass{article}
\usepackage{enumitem}
\usepackage{kantlipsum}
\setlist[enumerate]{nosep,itemindent=2em}
\newcommand{\trickindent}{%
\let\normalitem\item
\def\item{%
\itemindent=0pt
\normalitem
\itemindent=2em
\let\item\normalitem}%
}
\begin{document}
\begin{enumerate}
\item\kant*[1]
\item\begin{enumerate}[before=\trickindent]
\item\kant*[2]
\item\kant*[3]
\end{enumerate}
\end{enumerate}
\end{document}
\begin{document}